Method for image processing using local statistics convolution

ABSTRACT

A method for filtering a digital image, comprising segmenting the digital image into a plurality of tiles; computing tile histograms corresponding to each of the plurality of tiles; deriving a plurality of tile transfer functions from the tile histograms preferably using 1D convolutions; interpolating a tile transfer function from the plurality of tile transfer functions; and filtering the digital image with the interpolated tile transfer function. Many filters otherwise difficult to conceive or to implement are possible with this method, including an edge-preserving smoothing filter, HDR tone mapping, edge invariant gradient or entropy detection, image upsampling, and mapping coarse data to fine data.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is a divisional of U.S. application Ser. No.14/094,405 filed Dec. 2, 2013 and issued as U.S. Pat. No. 9,311,694 onApr. 12, 2016, which is a Continuation of U.S. application Ser. No.12/566,782 filed Sep. 25, 2009 and issued as U.S. Pat. No. 8,625,921 onJan. 7, 2014, which claims the benefit of U.S. Provisional PatentApplication No. 61/100,604 titled “Method for Image Processing UsingLocal Statistics Convolution” filed Sep. 26, 2008, the content of whichis hereby incorporated by reference in this disclosure in its entirety.

BACKGROUND

This disclosure relates to digital image filtering, based on applying 1Dconvolution kernels to 1D local histogram data, and is thereby verydifferent from the conventional idea of applying 2D convolution kernelsto 2D image data.

One major application of this image filtering domain is to enhance localcontrast in an image. One conventional approach of doing this is tospread the values of the image signal so that the lowest appearing valueis forced to 0.0 and the highest appearing value is forced to 1.0. Ifthis is done for the entire image, this is an old, well-establishedapproach. This can also be done locally, that is tile-by-tile, which isalso not a novel approach. Another approach tries to find a local upperbound and a local lower bound of the image signal, which can beunderstood as a locally dependent “lowest value” and a locally dependent“highest value”, so that the image signal is then spread so that thelower boundary is set to 0.0 and the upper boundary is set to 1.0. Thedownside is that this system creates artifacting and may take a longtime to process.

There is a high demand in the field of image processing for edgepreserving smoothing filters. An old, known filter of this kind is themedian filter, as it leaves strong edges in the image without blurringthem, while small details are erased. Another representative filter ofthe class of edge preserving smoothing is the bilateral filter, which isa derivation of the Gaussian blur filter. It has a wide usage forphotographic and analytic image processing. Its downside is that a fastand efficient implementation is not easily possible.

SUMMARY

The invention surmounts these deficiencies in the prior art by providinga method for using 1D-convolution of histogram data to construct fastand efficient image filters. The histogram-convolution based imageenhancements of the invention can be performed with minimal memory andprocessing requirements, and make extensive and fast use of a largerange of filters.

Presented herein is a system that we name bilateral histogramequalization, which is a histogram equalization that spreads the imagesignal to span a range from 0.0 to 1.0 within the context of localobjects in an image, that is for every pixel a lower boundary isestablished concerning only the peak of the local histogram that thispixel belongs to, and a higher boundary is established in similarfashion. Within the context of this disclosure, using 1D convolutions on1D histogram data, this can be done without explicitly determining saidboundaries; instead, an equation is provided in direct form thatdirectly treats the signal so that each object in the image spans acontrast range of 0.0 to 1.0. This is not merely a formal difference, italso leads to different results, as pixel neighborhood issues dont playa role in this approach, and the computation is also faster.

An advantage of the system disclosed herein is that an edge-preservingsmoothing filter can be implemented where, again, pixel neighborhoodissues play no role and therefore many artifact problems are eliminated.The EPS filter introduced in this disclosure does again function on theidea of applying 1D convolution kernels to 1D histogram data. In itsresults it compares to a bilateral filter, while the processing hereintakes place very fast, and full flexibility with respect to theconvolution in signal domain is possible, unlike other fast embodiments.

This disclosure also describes a large amount of other filters using thesaid philosophy of 1D convolution kernels on 1D statistical image datain order to create a Tile Transfer Function, also called a RegionTransfer Function, that can be used to enhance an image. Many of thesefilters would be difficult to conceive or to implement without thesystem disclosed herein. For instance, an edge-preserving smoothingfilter is easily implemented with this concept. Other applicationsinclude HDR tone mapping, edge invariant gradient or entropy detection,image upsampling, mapping coarse data to fine data, and more. Sincemostly 1D kernels are computed on histogram data, only little processingpower is required, so that all filters in this disclosure run in lineartime.

A method is disclosed for filtering a digital image comprising a firstplurality of pixels, comprising segmenting the first plurality of pixelsinto a first plurality of regions; computing a first plurality ofregional histograms by calculating a regional histogram corresponding toeach region in the first plurality of regions; computing a firstplurality of regional transfer functions by calculating for each regionin the first plurality of regions a regional transfer functioncomprising a convolution of the corresponding regional histogram with afirst convolution kernel; deriving an image transfer function byinterpolation of two or more of the first plurality of regional transferfunctions; and filtering the digital image by applying the imagetransfer function to each of the first plurality of pixels. The firstplurality of pixels may comprise the entire digital image. Optionally,the regions may be chosen so that they do not overlap, and each pixel iscontained within a single region.

In one embodiment, interpolation comprises selecting a pixel having aluminosity value from a first region of the first plurality of regions;computing a plurality of single value results by inputting the selectedpixel's luminosity value into each of a second plurality of regionaltransfer functions, said second plurality including regional transferfunctions corresponding to regions not comprising the selected pixel;and interpolating a resulting value for the selected pixel as a functionof the plurality of single value results.

The first convolution kernel could be a Gaussian function. Preferably,the first convolution kernel is one-dimensional.

The regional transfer function may also be a function of a secondconvolution comprising the convolution of the corresponding regionalhistogram with a second convolution kernel. Optionally, the firstconvolution kernel is a Gaussian function, and the second convolutionkernel is a product of a Gaussian function and a step function. The thesecond convolution may optionally be weighted by pixel luminosity.

The regional histograms may be function of luminosity, or of othercharacteristics such as pixel difference, color, depth along a z-axis,or of saturation.

Another embodiment for filtering a digital image comprising a firstplurality of pixels is provided, each pixel having a luminosity valueand being within a neighborhood comprising a second plurality of pixels,the neighborhood having an extent, comprising selecting a pixel from thefirst plurality of pixels; calculating a new luminosity value for theselected pixel as a function of the ratio of the number of pixels in theneighborhood that fulfill a given condition to the total number ofpixels in the neighborhood, the condition comprising a luminancecomparison against said pixel; and repeating the calculating step untilthe luminosity values of each of the first plurality of pixels has beencalculated. The first plurality of pixels could comprise the entiredigital image. In one embodiment, the extent of the neighborhood isdefined by a distance function, such as a spatial distance, a vectordifference, or a colormetrical distance. The pixel numbers in the ratiomay be weighted, such that pixels with a low distance to the selectedpixel receiving a high weight and pixels with a high distance to theselected pixel receiving a low weight.

A program storage device readable by a machine, tangibly embodying aprogram of instructions executable by the machine to perform methodsteps to transform a digital image comprising a first plurality ofpixels is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of one embodiment of the methods of theinvention.

FIG. 2 is a representation of a digital image in which a histogram hasbeen found for a portion of the image.

FIG. 3 is graph showing the relationship of the histogram to the edgepreserving filter.

FIG. 4 is a set of representations of digital images showing the resultof applying different filters.

FIG. 5 is a set of three representations of a digital image (left),showing the comparison of a prior art highlights filter (middle) and abilateral histogram equalization according to the invention (right).

FIG. 6 is an example of a three-dimensional histogram.

FIG. 7 illustrates how equation 30 can be imagined within the signaldomain.

DETAILED DESCRIPTION

1 Introduction

The general idea in this disclosure is based on separating the imageinto tiles, also called regions. These tiles can have dimensions of 5×5pixels up to ⅓rd of the image dimensions, depending on the application.For each of these tiles (or regions) a so-called tile transfer function,or region transfer function, is found so that if an image has 100 tiles,we will compute 100 local transfer functions. These functions are thenused to map, for instance, input luminosities to output luminosities. Toavoid a tiling effect (that is, visible edges along the margins of thetiles), the tile transfer functions need to be interpolated when theyare applied to the final image.

FIG. 1 illustrates this concept. In (1), The image I is separated intotiles I_(1,1) . . . I_(X,Y) . . . I_(MaxX,MaxY). In (2), for each tile ahistogram is computed. Then, in (3), for each histogram a tile transferfunction ƒ(x) is derived from the histogram. In (4), an interpolationtakes place between the tile transfer functions and the resulting pixelsare computed. For example, J_(x,y)=Interpolation(ƒ_(X−1,Y−1),ƒ_(X+1,Y+1)) (I_(x,y)).

Between step (2) and step (3), convolution kernels are applied to thehistograms in order to obtain the tile transfer functions. While inconventional image editing, 2D convolution kernels are applied to 2Dimage data, here 1D convolution kernels are applied to 1D histogramdata. This introduces new possibilities for image filtering, while thesame time much less processing power is required, since the convolutionis performed on a much smaller data set. In most cases, the calculationof the tile transfer functions requires less operations than the imagehas pixels, so the most processing intense steps are the calculation ofthe histograms (2) and the application of the tile transfer functions(4), which are both linear. Some of the filters introduced in thisdisclosure are an Edge Preserving Smoothing filter, a BilateralHistogram Equalization filter, useful as a HDR tone mapper, anEdge-Ignoring Entropy Detector, a filter to map coarse data onto finedata, and a filter to measure soft image gradients, invariant to strongedges.

The concept introduced herein is referred to as the LSC filter domain.LSC stands for “Local Statistics Convolution”; this term was chosenbecause the convolution of histogram data is an important concept forall of the described filters.

1.1 Terminology & Conventions

The following conventions are used herein:

-   I is the original image.-   İ and Ï are processing results of applying a filter to I.-   J is typically the resulting image.-   (x,y) denote a pixel coordinate, I_(x,y) and J_(x,y) denote pixels    within I and J.-   a will denote the edge length of an image tile, the tile size being    a×a.-   (X,Y) denote the coordinate of an image tile.-   i denotes a pixel intensity.-   h_(X,Y)( ) denotes a histogram of the tile at (X,Y). Sometimes we    will omit the indices, so that a tile histogram is noted as h( ).-   ƒ_(X,Y)( ) will denote a tile transfer function.-   {tilde over (ƒ)}_(x,y)( ) is an interpolated tile transfer function    for the pixel (x,y).-   Id stands for the identity function Id(t)=t.-   ◯ stands for the convolution operator.-   gauss stands for a Gaussian kernel, often being 1D in this    disclosure.-   blur stands for a blurring kernel, for instance a Gaussian or box    filter.-   ∇ is the gradient operator, being

$\frac{\delta\; I}{\delta\; x} \times {\frac{\delta\; I}{\delta\; y}.}$2 LSC Filtering

The histogram of an image

$\begin{matrix}{{h_{i} = {\sum\limits_{x,y}{\delta\left( {I_{x,y} = i} \right)}}};} & (1)\end{matrix}$contains cumulated data of all image features, that is, h_(i) providesthe amount of pixels of the intensity i. Note that δ( ) is an operatorevaluating to 1 if its expression is true, and zero otherwise. However,finding such a histogram h is often worthless, as one will find allfeatures of an image, being often hundreds of objects and shades,combined in one histogram.

However, when finding the histogram of a small portion in an image,let's sayI _(aX . . . aX+a−1,aY . . . aY+a−1)it is much more likely that the tile histogram will provide moreinsights, as the smaller histogram is more likely to contain only one,two, three or four objects. So the assumption can be made that tilehistograms in an image have fewer and more clearly defined peaks. A tilehistogram is shown in FIG. 2.

Let us now divide the image I into a set of sub-histograms h_(1,1),h_(1,2), h_(1,3) . . . h_(max) _(x) _(,max) _(y) so that h_(X,Y,i)provides the amount of pixels with the luminosity i in the tile X,Ywithin the image. We now have divided the image into tiles andcalculated histograms for each of the tiles. As these histograms arelocal, they contain only few statistical values of a local neighborhood.

2.1 the LSC Filter—Image Blurring

A basic LSC filter is applying a blur within the LSC filter domain,therefore it is a good introduction to LSC filtering. Let us define afunction ƒ_(average)(h_(X,Y)) that calculates the average of thehistogram h_(X,Y):

$\begin{matrix}{{f_{average}\left( {h_{X,Y},i} \right)} = \frac{\sum\limits_{j}{{jh}(j)}}{\sum\limits_{j}{h(j)}}} & (2)\end{matrix}$

Note in equation (2) that the cumulated values of h_(i) (the expressionunder the fraction) is no different from a², a being the edge length ofa local image tile.

As is easy to see, ƒ_(X,Y,i) is constant for i and varies only from tileto tile, providing an average for each tile contents. Let's replaceevery pixel in I with the respective value in ƒ_(X,Y):J _(x,y)=ƒ(┌x/a┐,┌y/a┐,I _(x,y))  (3)

This will lead, obviously, to a mosaicking effect, resulting in an imagewith one uniform gray level for each tile of a size of a×a pixels.However, performing a bilinear, Gaussian, bicubic or cosineinterpolation between the tiles will lead more or less to a blurry imagewith a large blurring radius of the magnitude of a.

2.2 EPS Filtering

Multiplying each histogram entry with its index (equal to multiplyingthe histogram with the identity function) and dividing it by the sum ofthe histogram entries leads to the average intensity. This constrainsthe focus to a bandwidth within the signal domain, resulting in anaverage value representative for a certain reference color i:

$\begin{matrix}{{f_{{EPS},X,Y}(i)} = {\frac{\left( {h \cdot {Id}} \right) \circ {gauss}}{h \circ {gauss}}(i)}} & (4)\end{matrix}$

Equation 4 above divides a blurred version of h·Id, that is, a histogramwhere its entries are multiplied with its index, by a blurred version ofthe histogram itself. Note that ◯gauss denotes a 1D-convolution with aGaussian filter. Note that equation 4 is in its idea identical toequation 2, with the difference that the average calculated for every iis constrained within the signal domain.

Applying equation 4 to every pixel in the image will result in a detailreduction per tile. We'll from here on call functions like ƒ_(EPS) atile transfer function, or briefly TTF. This function, if applied toeach pixel in the tile, will reduce details. As shown in FIG. 3, ƒ_(EPS)(dashed line) plateaus where h( ) (solid line) has peaks, and the centerof these peaks coincides with the identity (dotted line). This basicallymeans that image areas belonging to one peak are smoothed by the TTF,where the average of the original values is about identical to theaverage of the smoothened areas.

However, it goes without saying that this leads to strong tilingartifacts. Prior art solutions include calculating a local histogram forevery pixel, while the present invention performs an interpolationbetween the tile transfer functions.

Given a pixel x, y, the tile edge length a, an image I, a set of tiletransfer functions ƒ_(X,Y)(i), then an interpolation could for instancebe:

$\begin{matrix}{{{\overset{\sim}{f}}_{x,y} = {\sum\limits_{X,Y}{f_{X,Y} \cdot {\omega\left( \frac{\sqrt{\left( {x - {aX} + \frac{a}{2}} \right)^{2} + \left( {y - {aY} + \frac{a}{2}} \right)^{2}}}{a} \right)}}}}{with}{{\omega(d)} = {\frac{1}{\sigma\sqrt{2\;\pi}}{\exp\left( {{- \frac{1}{2}}\left( \frac{d}{\sigma} \right)^{2}} \right)}}}} & (5)\end{matrix}$

Obviously, a Gaussian interpolation is provided above with sigma beingthe standard deviation, while the unit of the standard deviation in thiscase is not pixel size, but tile size. We found that from all possibleinterpolations performed between tile transfer functions:

-   -   bilinear works fast and well, but not superb    -   cosine can create artifacts in soft gradients    -   bicubic works mediocre    -   Gaussian (see equation 5) works optimal, even if interpolation        takes place on as few as 5×5 or even 3×3 tiles.

We find that bicubic does not work optimally as said interpolationattempts to maintain optimal detail through the interpolation, which isnot wanted here, since a smoothing effect is desired. Therefore wesuggest Gaussian interpolation.

We denote ƒ_(X,Y)( ) as the TTF of a given tile (X,Y), while we willdenote {tilde over (ƒ)}_(x,y)( ) as the already interpolated functionthat varies from pixel to pixel. Note that when Gaussian kernels areused for the histogram convolution as well as for the spatialinterpolation, ƒ_(x,y)(i) will be smooth with respect to variations ofx, y and i.

The process shown herein provides a fast way for EPS filtering at aspeed of O(n), since the creation of the histograms h_(X,Y) requires asingle pixel-read operation for each pixel in the image. Creation of thetile transfer function TTF, for instance as in equation 4, requiresalmost no processing time, as it is performed on 1D data, which istypically much less data than the image data. Finally, the values foundin the TTF of 2×2, 3×3 or 5×5 adjacent tiles need to be interpolated,which consumes the most processing time, but still can be performed inlinear time. (Even large softening radii can be simulated by increasinga as opposed to increasing σ) Note that while equation 5 suggests forevery pixel in the image the interpolation of the entire function, ofcourse only a single element of the function needs to be interpolated.

Since large as well as small radii can typically be simulated with aninterpolation between a constant number of tiles, the processingrequirements are linear, that is, regardless of image size andconvolution kernel a complexity magnitude of O(n) can be achieved wheren is the amount of pixels in the image. LSC processing will only exceedthese limits when a very high resolution of the histograms in theintensity domain is desired, so that the histogram entries exceed thenumber of pixels within a tile, but those appear to be rare cornercases.

2.3 Generalization of the LSC Concept

In general, the idea of LSC filtering is now as follows:

-   -   separate the image into tiles of dimensions a×a;    -   calculate local statistics (e.g., a histogram) for each of said        tiles;    -   derive TTF from the local statistics of each tile, for instance        by applying convolution    -   filters to the histograms;    -   interpolate between TTF of adjacent tiles; and    -   apply the interpolated TTF to the image.

As introduced above, this allows, for example, for fast EPS filtering.However, the way the statistics are collected per tile (1D as shown, or3D for color images) can be varied, and of course the creation of theTTF provides as well for many options. The following sections willintroduce more applications of LSC filtering.

2.4 Rarity Operator

The LSC framework provides for a fast method to determine how “rare” apixel's value is within its neighborhood. While determining the rarityof a single pixel color within an image is easy, determining the samevalue for all pixels within a certain neighborhood in efficient time andbased on robust statistics isn't. With the following TTF:

$\begin{matrix}{{f_{rarity}\left( {X,Y,i} \right)} = {\log\frac{1}{\left( {h_{X,Y} \circ {blur}} \right)(i)}}} & (6)\end{matrix}$

where the term (h_(X,Y)◯blur)(i) is to be understood as the i'th valueof the function h_(X,Y) convolved with a blurring kernel, will provide avalue for how rare the given pixel's value is within a spatial range ofa (see equation 5) while a tolerance within the signal domain isprovided by the standard deviation of the kernel blur.

2.5 Non-Edge Sensitive High Pass Filter

The following TTF:

$\begin{matrix}{f_{{highpass},X,Y} = {{Id} - \frac{\left( {h \cdot {Id}} \right) \circ {gauss}}{h \circ {gauss}}}} & (7)\end{matrix}$

provides a filter for extracting the high pass out of an image whilestrong edges are ignored. Typically, this will extract the structure ofan image (fine branches in a tree, fur of an animal, fine cloudstructures) while not providing extreme values for strong edges (likethe transition from a building to the sky). Similarly, the tile transferfunction

$\begin{matrix}{f_{{increasestructure}\;,X,Y} = {{Id} + {\gamma\left( {{Id} - \frac{\left( {h \cdot {Id}} \right) \circ {gauss}}{h \circ {gauss}}} \right)}}} & (8)\end{matrix}$

will emphasize the image's structure by the factor provided in γ. FIG. 4shows a digital image (top left) filtered with an EPS (top right), theresult of equation 7 (bottom left) and the result of equation 8 withγ=0.8 (bottom right).

2.6 Entropy Filter

Determining the entropy of an image is easy, and many local entropyoperators have been suggested. However, to our knowledge, no entropyfilters are known that are edge-preserving. That is, most entropyfilters analyze an image area of n×n pixels around a given pixel x, y,determine the entropy of that area, and assign said entropy to thecoordinate x, y. This will lead to the effect that a small patch of bluesky that's surrounded by branches of a tree will have a high entropy,while a remote area of the same sky will have a low entropy. Sometimes,this will be the desired behavior, sometimes it would be desired thatall pixels of the sky lead to about the same entropy. The following tiletransfer function can be used:ƒ_(entropy,X,Y)(h _(X,Y) ,i)=σ_(i)  (9)with σ_(i) being the standard deviation of:

$\begin{matrix}{{g(j)} = {{h(j)}*{\exp\left( {{{- 1}/2}\frac{\left( {j - {\mathbb{i}}} \right)^{2}}{\psi}} \right)}}} & \;\end{matrix}$where ψ is the standard deviation of the superimposed filter, so thatgenerally σ_(i)≦ψ. This means that ψ defines the ‘window’ in which anentropy can be detected, that is:

-   -   no variances in a structure (plain sky) will lead to a low        entropy    -   variations in a structure within the magnitude of ψ will lead to        a higher entropy    -   variations in a structure greater than the magnitude of ψ (such        the black letters on white paper) will not affect the entropy.        2.6.1 Alternative Entropy Finding

An alternative approach to finding an entropy is as follows:

$\begin{matrix}{{{h(i)} = {\sum{\delta\left( {I_{x,y} = i} \right)}}}{{k(i)} = {\sum{{u\left( {{I_{x,y} - I_{{x + 1},y}}} \right)}*{\delta\left( {I_{x,y} = i} \right)}}}}{f_{{entropy}\mspace{11mu} 2} = \frac{k \circ {blur}}{h \circ {blur}}}} & (10)\end{matrix}$

In equation 10, sums of pixel neighbor differences are divided by theamount of pixels of that color, where both again are convolved with a 1Dblurring filter. The function u( ) can be used to dampen pixeldifferences that are likely to be hard edges in the image and unlikelyto be structure-related pixel differences, where u( ) can be a weightingfunction, such as u(x)=√{square root over (x)}, or u(x)=x in the trivialcase.

2.6.2 Usages for Entropy

The Entropy can be used to modify an image. For instance, the image canbe made more pleasing by lightening those areas that have a high entropyand darkening the others. Imagine a person being photographed against asky, where the sky has a low entropy and the person has a highentropy—accentuating the person by increasing his/her brightness whiledarkening the background will make the image more pleasing.

Conventional entropy filters will not be able to perform this task, asthey'd add a soft, large, defocused glow to the edges within the image(remember that conventional entropy filters detect a high entropy a fewpixels to either side of dominant edges), so the technique of changingthe illumination based on entropy will only work if the entropy isdetected in an edge-preserving fashion (as suggested in here).

2.7 Local Histogram Equalization

The concept of histogram equalization is well known. The idea is toredistribute the color intensities in an image with the formula:

$\begin{matrix}{{f(r)} = {\frac{\int_{0}^{r}{{h(i)}{\mathbb{d}i}}}{\int_{0}^{1}{{h(i)}{\mathbb{d}i}}} \cdot 256}} & (11)\end{matrix}$

If ƒ( ) is being applied to all pixels in the image, each luminositywill appear with the same likelihood in the image. This is sometimes awanted effect, sometimes this effect has disastrous side-effects to theimage—particularly when one gray level is dominant (like a plain whitesurface), whose intensity values will then be distributed over a widerange of values, even if the image resolution (resp. dynamic range)doesn't provide a sufficient PSNR for this kind of operation.

The equation:

$\begin{matrix}{{f(r)} = {\frac{\int_{0}^{r}{{u\left( {h({\mathbb{i}})} \right)}{\mathbb{d}i}}}{\int_{0}^{1}{{u\left( {h({\mathbb{i}})} \right)}{\mathbb{d}i}}} \cdot 256}} & (12)\end{matrix}$

will not have these deficiencies. The limiting function u( ) is definedas earlier in this document, for instance, u( ) can be a square root.This will ensure that the equalization will affect mostly those areaswith low values in the histogram; in other words: the function abovewill modify the histogram so that gaps between the histogram's peakswill be filled up, while peaks remain peaks of about the same shape,leading to more natural structures.

Now applying equation 12 tile-wise as shown in equation 2 (calculatingtiles of histograms) and 5 (interpolating between transfer functions)will result to a locally adaptive histogram equalization. Note that wesuggest for better effects a large tile size in the image, such as10%-20% of the image dimensions.

2.8 Bilateral Histogram Equalization

The implementation as shown in equation 12 will have a haloing issue,which could be avoided by using very large tiles. (Halos of a magnitudeof 33% to 50% of the image dimensions are not perceived as halos by thehuman eye). However, this limits the filter's ability to enhance imageslocally, particularly counter-light effects of a magnitude smaller than33%-50% of the image size will not be corrected. So a smaller tile sizeis desired while not producing halos along strong edges.

It is preferable to introduce a bilateral dependency, noted as:

$\begin{matrix}{{f(r)} = {\frac{\int_{0}^{r}{\left( {{u(h)}{\exp\left( {{- 1}\text{/}2\frac{\left( {{\mathbb{i}} - r} \right)^{2}}{\sigma}} \right)}} \right)({\mathbb{i}}){\mathbb{d}i}}}{\int_{0}^{1}{\left( {{u(h)}{\exp\left( {{- 1}\text{/}2\frac{\left( {{\mathbb{i}} - r} \right)^{2}}{\sigma}} \right)}} \right)({\mathbb{i}}){\mathbb{d}i}}} \cdot 256}} & (13)\end{matrix}$

which is similar to:

$\begin{matrix}{{f(r)} = {\frac{{u(h)} \circ {leftgauss}}{{u(h)} \circ {gauss}} \cdot 256}} & (14)\end{matrix}$

where the magnitude (σ) of the Gaussian bell curve used for convolutionneeds to be of a high magnitude, so for instance if images with valuesreaching from [0 . . . 255] are given, a σ of 50 . . . 150 may beadequate. The convolution kernel leftgauss is meant to be a Gaussiankernel where all values to the right of its center are forced to zero.This can be done by a product of the Gaussian function and a stepfunction with zero for all positive values. Of course, in analogousfashion a rightgauss kernel could be constructed with a step functionwith zero for all negative values, and then inverted.

The idea of the above given equations is to detect for a given pixel'sluminosity i if it belongs to the lower, middle or higher part of itspeak in the histogram. The Gauss functions are used to fade out remote,unwanted peaks, and then the integral of the area to the left of i isdivided by the entire integral. That provides information how the inputluminosity i relates to the luminosities of the entire peak.

This is to say that equations 13 and 14 are examples of functions thatmay be used to calculate a new luminosity value of a selected pixel as afunction of the ratio of the number of pixels in the neighborhood thatfulfill a given condition to the total number of pixels in theneighborhood. The condition in this case comprises a luminancecomparison, in which the function selects for pixels that are brighter(or darker) than the selected pixel.

The neighborhood could be chosen as appropriate, and the extent of theneighborhood could be defined with a distance function. The distancefunction could comprise one or more of a spatial distance, a vectordifference, or a colormetrical distance. The distance function could beused to weight the number of pixels, so that pixels with a low distanceto the selected pixel receive a high weight and pixels with a highdistance to the selected pixel receive a low weight. Note that in theLSC system, the convolution of the histogram data with a 1D kernel isone fast way to weight pixels based on the abovementioned distance withrespect to luminance. All or part of the image could be processed byusing these equations repeatedly.

This also is a very good HDR tone mapping operator, as its results lookoften natural and as the TTF automatically maps the input signal to apredefined range, regardless what the distribution of the input signalsis. FIG. 5 shows a digital image (left) that has been filtered with aprior art highlights and shadows correction (center), and a bilateralhistogram equalization according to the present invention (right).

A fundamental difference between this HDR tone mapper and the majorityof prior art tone mappers is that conventional tone mappers try torelate a pixel to its surrounding average luminosity, namely the“adaption luminance.” Said adaption luminance is a scalar value andcarries no information about the local structure. This tone mapper putseach pixel not only into relation to an average luminance, but inrelation to the entire surrounding peak, including its center and itsbroadness.

2.9 Artifact Free Depth-Map Image Blurring

Imagine an image with a candle in front of a patterned wall, the candlebeing at a distance of 5 cm from the camera, the wall being at 15 cmfrom the camera, and further imagine this image to be recorded with a 3Dcamera recording depth information. So, in addition to the colors red,green, and blue, there would be a depth map D_(x,y) showing low valuesfor the candle and high values for the wall, representing distances fromthe camera.

If now the task is to blur the background behind the candle, the mostconvenient way would be to blur the image and to replace all pixels witha depth value D_(x,y)=15 cm with the blurred image. However, this willcreate a halo around the candle, since part of the blurry candle will bewritten into the background.

We believe this issue can be solved with the LSC approach. An option isto create for each image tile [X,Y] a histogram that doesn't map pixelcount based on luminosity, but on depth. Subtotal, we will need thefollowing histograms for each tile (we omit the tile indices X,Y):

$\begin{matrix}{{h_{R}(d)} = {\sum\limits_{x,y}{I_{R,x,y}{\delta\left( {D_{x,y} = d} \right)}}}} & (15)\end{matrix}$

h_(G)( ) and h_(B)( ) can be calculated in a similar way.

$\begin{matrix}{{C(d)} = {\sum\limits_{x,y}{\delta\left( {D_{x,y} = d} \right)}}} & (16)\end{matrix}$

The three functions h_(R), h_(G) and h_(B) hold now accumulatedluminances, categorized by the depth value d, where the histogram Cholds accumulated pixel counts per tile, categorized by the depth valued.

Now a blurred version of the image can be processed by the followingTTF:

$\begin{matrix}{{f_{{depthblur},R,X,Y}(d)} = {\frac{{\left( {h_{R} \circ {gauss}} \right)(d)} + \varepsilon}{{\left( {C \circ {gauss}} \right)(d)} + \varepsilon}.}} & (17)\end{matrix}$

The function ƒ_(depthblur,R,X,Y) will now produce a blurred value forthe red channel for the current tile X,Y for each depth d. Thecalculation of the green and blue channel is equivalent. For instance,if the current tile X,Y may contain a red candle at 5 cm distance fromthe camera, while containing a blue background at 15 cm from the camera,then ƒ_(depthblur,X,Y)(5) will deliver the average of the candle's redpixels, while ƒ_(depthblur,X,Y)(15) will deliver the average of thebackground.

Note that the spatial blurring radius will be determined by theunderlying tile size a×a, while the Gaussian filter provided in equation17 will determine how far pixels have to be away from one another alongthe 3rd dimension D_(x,y) in order to not be convolved with one another.So if in our example the background should not be mixed with the candle,the Gaussian kernel gauss should extend over less than 10 cm. Note thatin this example very small tile sizes can be used, for instance a=5 toachieve not-too-large blurring radii, in which case it will be likelythat C( ) is a very sparse histogram. For that case, ε in equation 17avoids division by zero.

Applying the TTF ƒ_(depthblur)( ) to the image as shown in equation 5will lead to a blurred image where objects close to the camera will not‘smear’ into objects far away from the camera during the blurringprocess. This image can then be blended with the original image based onthe depth map D_(x,y) where the above described artifacts should notoccur.

Note that the size of the Gaussian kernel in equation 5 can be variedbased upon the current pixel's depth D_(x,y) while the Gaussian kernelin equation 17 may be varied, for instance to achieve that objects at 5cm and 15 cm distance from the camera will not blend into one another,while objects at 105 cm and 115 cm distance from the camera actually doblend into one another.

2.9.1 More Observations to Depth-Maps

Let us examine some properties of the 1D convolutions. The Gaussiankernel does not drop to zero, that is the Gaussian bell curve isalways >0. (Note that this is not the case with many pseudo-Gaussianfilters which are often used for speed reasons). This characteristic ofthe Gaussian function means that sparse, positive datasets will also notcontain any zeros after being convolved with a Gaussian function. Ofcourse, this can only be observed when floating point numbers are usedin the implementation.

This has some interesting implications. The first implication is thatthe tile size can be reduced dramatically. For instance, with a tilesize of 4×4 pixels, the histogram won't have more than 16 entries,256-16 or more will remain empty. However, even if such a sparsehistogram is used, functions like ƒ_(EPS) or ƒ_(3DBlur) will be definedfor the domain of [0 . . . 255] and they will be smooth. Therefore thefilters herein work as well with small tile sizes.

The second implication is that all tile transfer functions suggested inthis disclosure can be evaluated for parameters not occurring in theimage. So for instance, ƒ_(depthblur) can be evaluated for everylocation in the image and for every 3D depth D. This means that we havea volumetric information for every spatial position of the imagesuggesting a “most likely color.” For instance, imagine again an imagewith a red candle in front of a blue wall, the candle at 5 cm distanceand the wall at 15 cm distance, and imagine that the pixel coordinate(100,100) is in the middle of the candle. Then evaluating the functionƒ_(depthblur)(100, 100.5 cm) will, obviously, lead to a red value, whileƒ_(depthblur)(100, 100, 15 cm) will provide a color that is assumed tobe behind the candle, in this case most likely blue.

This allows us to determine for every location within the 3D space overthe image a most likely color. This can be helpful to complete a 3Dscenario that has been recorded incompletely due to the limitedperspective of the camera.

Additionally, we can make an assumption on where likely surfaces are.(For instance in order to receive an assumption on the surface data ofthe wall behind the candle where it was not recorded by the camera). Letus define:

$\begin{matrix}{{h_{pixelcount}(d)} = {\sum\limits_{x,y}{{\delta\left( {D_{x,y} = d} \right)}.}}} & (18)\end{matrix}$

Then we receive:

$\begin{matrix}{{f_{{depthdensity},X,Y}(d)} = {\frac{\left( {h_{pixelcount} \cdot {Id}} \right) \circ {gauss}}{h_{pixelcount} \circ {gauss}} \cdot {(d).}}} & (19)\end{matrix}$

Now while h_(pixelcount) holds 1D statistics of pixel count versus depthper each tile, the function ƒ_(depthdensity,X,Y) has plateaus whereƒ_(pixelcount) has peaks, and it is smooth. Particularly, {tilde over(ƒ)}_(depthdensity,x,y) (the interpolated version, see above) is smoothwithin spatial and depth domain.

Now the expression{tilde over (ƒ)}″_(depthdensity,x,y)=0 and {tilde over(ƒ)}′″_(depthdensity,x,y)>0  (20)

defines a manifold that will be a smooth surface within the 3D spacethat does not have holes and that will suggest a location of a surfacewithin the 3D space even for those locations in the image that were notrecorded, e.g., the wall behind the candle, or the backside of thecandle. Note, though, that the precision of the routine is limited to a,a being the tile size.

Note that a different definition would be (h_(pixelcount)◯gauss)′=0 and(h_(pixelcount)◯gauss)″<0, which is longer to write down but a littleeasier to implement.

Now with equations 20 and 17, we will have information forlikely-to-exist surfaces and for most-likely colors at the location ofthose surfaces. This will allow us to rotate the recorded 3D image by afew degrees and to inpaint the then missing regions.

2.10 Mapping Imprecise Information to High Resolution Data

It often occurs that a camera captures good luminosity data butadditional data (chrominance, 3D depth, infrared temperature, etc.) witha lower spatial resolution. We call the former fine data and the latterrough data. For many purposes it might be desirable to upsample therough data to the resolution of the fine data while reproducing the finedata edges in the upsampled channel.

Let I_(x,y) ^(fine) denote the fine image data and I_(x,y) ^(rough) thetrivial upsampled rough channel. In the following we only considercoordinates x, y in the fine resolution. As in the above the image isdivided into tiles. Let

$\begin{matrix}{{r({\mathbb{i}})}:={\sum\limits_{x,y}{{\delta\left( {I_{x,y}^{fine} = {\mathbb{i}}} \right)} \cdot I_{x,y}^{rough}}}} & (21)\end{matrix}$denote the function taking fine channel intensities to therecorresponding cumulated rough channel intensities and

$\begin{matrix}{{f_{f\; 2r}({\mathbb{i}})}:={\frac{r \circ {gauss}}{h \circ {gauss}}({\mathbb{i}})}} & (22)\end{matrix}$denote a TTF taking fine data intensities to rough data intensities. Asin the previous sections the tile indices are omitted for the sake ofreadability. ƒ_(ƒ2r) might be understood as the average rough channelvalue belonging to a given fine channel value on a specific tile.Equation (22) is very similar to (4). The only difference is the wedistribute averaged rough channel data instead of averaged fine channeldata.

Using (22) as a TTF in (5) results in a new channel J_(x,y)approximating the values of I^(rough) and the edges of I^(fine).

Applications can be many:

(1) Imagine a camera that captures mainly luminosity information (i.e.,sensor elements with no color filter connected) and only sparse colorinformation. Then the rough chrominance data can be mapped to the fineluminance data.

(2) Imagine a camera that records a natural image with a fine resolutionand 3D depth data with a rough, poor resolution; then equation 21 can beused to map the poor 3D data to the fine color/luminance data.

(3) A camera recording other spectral information (infrared, UV) withlower precision than the visible light bands can use said algorithm tomap the rough data to the fine data.

(4) Conversely, if a camera records precise information in a non-visibleband, this band can be used to map the rough, visible data to the finedata.

“Roughness” does not necessarily need to mean lower resolution, it canalso mean that the rough data have a greater expected signal error(noise) or spatial error (distortion), or all of the three.

2.11 Edge Invariant 3D Gradient Field

It is theoretically possible to find detect surface orientation within3D space from looking at the gradients in a 2D image. However, nature isfar too complex for these simple assumptions, so real 3D construction inreal-word conditions is not really possible.

However, some simple image processes (such as pseudo-relighting) can beperformed by using the image's luminosity gradient instead of the 3Dgradient, that isg _(x,y) =∇I _(xy)  (23)

However, imagine an image with a ball in front of a candle. Both theball and the candle will have smooth gradients, but along the edgebetween ball and candle the gradient ∇I is quite extreme due to abruptimage changes. Typically, this is a gradient that is present in theimage, but that we do not want to obtain. While things are simple in animage with a ball and a candle, one will receive large amounts ofgradients that are undesired if images get more complex (trees, clouds,eye lashes, hair), that is most real world images.

So it would be beneficial to find an algorithm that spotssoft/continuous gradients within the image while ignoring edges betweenobjects.

Let's take an image I, being separated into tiles of dimensions a×a,while the tiles may be smaller here than in the preceding examples, suchas 15×15 pixels. Now we can make the following assumptions:

-   -   In each tile, there are about one to three different image        objects    -   Each image object has, within a tile, about one level of        luminosity    -   If adjacent tiles have same clusters of gray levels, these        belong to the same object    -   If adjacent tiles have similar, but slightly different clusters        of gray levels, this is an indication for a gradient.

Observing the TTF ƒ_(EPS) in equation 4, we find that ƒ_(EPS)(i)provides the average luminosity of the peak in the histogram that ibelongs to. Therefore,ƒ_(gradient)(X,Y,i)=∇ƒ_(EPS)(X,Y,i)dXdY  (24)

provides a measure for the gradient for a given luminosity in a givenimage tile. Note that the gradient operator is being calculated indirection of X and Y, the smallest discretization is the tile size, soonly gradients of the magnitude of a will be detected. (Of course, onecould detect the gradient based on {tilde over (ƒ)}_(EPS) which isdefined for each pixel, but that would provide no finer data, since theminimum wavelength occurring in all {tilde over (ƒ)} is a.) The sametime, a gradient will be provided for every tile and for everyluminosity i, even if a tile doesn't contain all possible luminosities.So in order to detect the gradient of a given pixel somewhere in theimage, ƒ_(gradient)( ) needs to be evaluated applying the LSCinterpolation methods as shown in equations 5.

2.12 Upsampling Using LSC

Imagine a low resolution of an image with black letters in front ofwhite paper. Common techniques to upsample an image (for instance todouble the image's spatial resolution in X and Y direction) includeadding additional pixels for every original pixel in the optimal case,typically even more. Typically, if interpolation takes place betweendark and bright pixels, interpolated pixels will often be of a graylevel between dark and bright. This will lead to many gray pixels alongthe edges of detail, so that the image will be perceived as being blurryafter the up-sampling. Some enhancements to this have been suggested,but many of the enhanced upscaling routines are slow or their resultsinclude blurriness, halos, or both.

We note I as the input image, İ as a conventionally interpolated image(for instance bilinear), İ having more pixels than I, and Ï our desiredresult, that is an image of the same dimensions of İ but with less of ablurry-edges effect. If (x,y) is a coordinate in I, (x′,y′) is thecorresponding coordinate in İ and Ï. (Note that in the realm of imageupsampling, it is common to deal with fractional pixel coordinates.) Ïcan then be computed as follows:Ï _(x′y′)=ƒ_(EPS(I))(İ _(x′,y′))  (25)

where ƒ_(EPS) needs to be derived from the original image I as inequation 4. This will ensure that only the original image's pixels withthe coordinate's (x,y) will contribute to the TTF ƒ_(EPS), so thatƒ_(EPS) will plateau along the peaks of the original image's histogram.Therefore, if ƒ_(EPS) is applied to the newly interpolated pixels of I,gray-level pixels will be transformed towards the peaks dominant in theoriginal image.

Experiments have shown that in this scenario, a small tile size producesgood results. Also, when tile sizes are around 5×5 pixels, aninterpolation as proposed in Equation 5 is not even necessary, as theresulting tiling artifacts are barely visible. Note, though, that theconvolution that needs to be performed for each tile as in equation 4will consume some processing power. This can be accelerated bydiscretizing the histogram to about only 30 levels of gray, where domainof the histograms (and therefore of ƒ_(EPS)) is limited to the pixelsactually present in said tile. Often the histograms need surprisinglyfew levels of discretization.

3 Enhancements to LSC Filtering

This section deals with enhancements to the LSC filtering that apply tomost of the filters introduced in section 2.

3.1 Color Dependent LSC Filtering

Instead of creating 1D-Histograms with only a luminance dimension, wealso can create histograms that are three-dimensional, such as <L*a*b>instead of just luminance.

FIG. 6 shows such a three-dimensional histogram, where pixel counts arenot mapped solely against their luminance, but against Luminance and twochrominances. Convolution needs to take place in three directions, whichdoesn't introduce any additional magnitudes of processing power, sincethe Gaussian filter is separable. However, due to memory efficiency wesuggest to limit the dimensions of a 3D histogram to 6-12 entries forthe color directions and to 20-100 entries for each luminance. Then, toavoid artifacting, during readout of the histogram a bilinearinterpolation in three directions is suggested.

Most equations in this document will work with three-dimensionalhistograms accordingly. Note that in a multiplication like Id·h( ) eachentry of the histogram will need to be multiplied with the pixel'sluminosity, disregarding chrominances.

Replacing 1D histograms with 3D histograms has many benefits. Basically,TTF like ƒ_(EPS)( ) or equations 13, 9 and 10, are capable of putting a“‘focus”’ on a certain band of luminosity. In the example of ƒ_(EPS), anaverage value for each band of luminosity is found (the band beingdefined by the 1D gauss filter). This can be refined by using a 3Dhistogram. Then the operations work on a local neighborhood as well ason a confined color space, being confined by luminance and chrominance.

One of the benefits thereof is that the TTF will be able to performa afiltering operation specific to one color, region and luminosity, whileignoring other colors in the same region and of the same luminosity. Forinstance, if equation 13 is implemented with a 3D histogram, the local,bilateral equalization can be performed individually for the blue skyand individually for the green meadow, even if both have the sameluminosity. Or with respect to entropy finding, if a 3D histogram isused, the presence of same-luminosity but differently colored objectswill not affect the detection of the entropy of a given object.

3.2 Plateau Sensitive LSC Filtering

If it is known that the image is blurry, then—let's say—a hair willconsist of a dark center and a gray, blurry margin. This margin is not awanted image area, as it is not representative for image contents, it'sonly representative for the image's blurriness. Therefore we suggestthat the histograms be weighted with

$\begin{matrix}{\kappa_{x,y} = {\log\left( \frac{1}{{\nabla\left( I_{x,y} \right)}} \right)}} & (26)\end{matrix}$

where kappa is a measure for the likelihood that an image pixel's coloris representative of an image feature opposed to a blurry edge betweentwo features.

3.3 Gradient Constrained Post Filtering

The idea of gradient constrained post filtering is based on theobservation that functions such as ƒ_(EPS) or in equation 11 can havesections where ƒ′(x)<ε including even ƒ∝(x)<0. In those cases imagestructures may get lost or turned into negative. This may lead to a lossof structure at local areas within the image, these areas rarelyexceeding a few percent of the total image area.

This can be fixed by identifying above described areas and ensuring thatthe resulting image J has at least a n-level-step gradient, while n isthe original image's gradient constrained to the bounds of [−4 . . . 4],for instance. Since this is a weak condition, this refinement does notrequire but high-frequency adjustments in the image, it is typicallyvery local, and therefore the gradients can be fixed by ensuring thiscondition to be true first line-wise, then column-wise.

In other words, what's needed is an algorithm that processes the imageline by line and determines if unwanted flat structures have occurred,and if so, forces the pixel differences to be at least the one of theoriginal image, or at least 4 levels of luminosity change in the samedirection, whichever is lower, and repeat the same for every column.

Another solution is to process the TTFs value by value and, whileprocessing each TTF, ensuring that the derivative of the TTF is withinan interval of [a, b], such as [0.25,4]. This proves to be a helpfulmethod to avoid extreme or negative contrasts. As the results may be(unwantedly) different depending on whether the TTF is passed fromleft-to-right or from right-to-left, it is preferable to compute theresult in both directions and interpolate in between the two results.Typically, all TTFs of an image have much fewer entries than the imagehas pixels, so processing resources are not an issue.

4 Comparison of LSC Filter Domain to Other Image Domains

We use the language LSC Filter Domain to describe the class of thefilters found herein. Unlike filters being applied to images' Fouriercomponents (Frequency Domain), wavelet components (Wavelet Domain) orgradients (Gradient Domain), the filters provided in this disclosure arenot being applied to the image in a different, bijective representationof itself. Therefore the conventional model of (1) transforming theimage into a complete, different representation of the original image,(2) applying filters to the coefficients of the new representation, and(3) converting the image back does not hold true for the LSC filterdomain. However, we still decided to name the concept provided in thisdisclosure a “filter domain,” since the suggested technique allows usto:

-   -   express filters based on a different domain (a 1D field of        accumulated pixel values versus a 2D field of frequencies,        gradients or signal intensities)    -   find short expressions for filters that would be complicated,        in-comprehensive in signal domain    -   use a new mind-set for filters that facilitates the creation of        new filters. (For instance, the filter provided in equation 24        would be very difficult to come up with in the spatial image        domain).

Therefore, we name the concept found herein a filterdomain, while thisdoes not mean that it is based on a complete image transform.

Often times, a parallel expression for a filter in signal domain can befound for expressions in the LSC filter domain. The following LSC filterdomain TTF:

$\begin{matrix}{f = \frac{\left( {h \cdot {Id}} \right) \circ {gauss}}{h \circ {gauss}}} & (27)\end{matrix}$

which is identical to equation 4 would need to be expressed in spatialdomain as:

$\begin{matrix}{I_{x^{\prime},y^{\prime}}^{\prime} = {\sum\limits_{x,y}{I_{x,y}{g\left( {{\left( {x,y} \right) - \left( {x^{\prime},y^{\prime}} \right)}} \right)}{{g\left( {{{I\left( {x,y} \right)} - {I\left( {x^{\prime},y^{\prime}} \right)}}} \right)}.}}}} & (28)\end{matrix}$

Also, the LSC domain filter TTF:

$\begin{matrix}{f = {\frac{h \circ {leftgauss}}{h \circ {gauss}} \cdot 256}} & (29)\end{matrix}$

which is a simplified version of equation 13, is similar to thefollowing expression in spatial domain:

$\begin{matrix}{I_{x,y}^{\prime} = \frac{\begin{matrix}{\sum\limits_{x^{\prime},{y^{\prime}{\forall{{I{({x^{\prime},y^{\prime}})}} \leq {I{({x,y})}}}}}}{{I\left( {x,y} \right)}{g\left( {{\left( {x,y} \right) - \left( {x^{\prime},y^{\prime}} \right)}} \right)}}} \\{g\left( {{{I\left( {x,y} \right)} - {I\left( {x^{\prime},y^{\prime}} \right)}}} \right)}\end{matrix}}{\begin{matrix}{\sum\limits_{x^{\prime},y^{\prime}}{{I\left( {x,y} \right)}{g\left( {{\left( {x,y} \right) - \left( {x^{\prime},y^{\prime}} \right)}} \right)}}} \\{g\left( {{{I\left( {x,y} \right)} - {I\left( {x^{\prime},y^{\prime}} \right)}}} \right)}\end{matrix}}} & (30)\end{matrix}$

with g( ) being a Gaussian bell function. As it is easy to see, theexpressions in spatial domain (eq. 28, eq. 30) are not only lessconvenient to discuss and to comprehend than the expressions in LSCfilter domain (eq. 27, eq. 29), they are also very processing intense tocompute in spatial domain. FIG. 7 illustrates how equation 30 can beimagined within the signal domain: The average luminosity of the lowerhalf of the depicted sphere is divided by the average luminosity of allpixels within the sphere. Here, the pixels are to be understood as a“particle cloud,” where a position of a pixel is defined by its spatialcoordinates and its luminosity.

All features disclosed in the specification, and all the steps in anymethod or process disclosed, may be combined in any combination, exceptcombinations where at least some of such features and/or steps aremutually exclusive. Each feature disclosed in the specification,including the claims, abstract, and drawings, can be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

This invention is not limited to particular hardware described herein,and any hardware presently existing or developed in the future thatpermits processing of digital images using the method disclosed can beused, including for example, a digital camera system, or a phone enabledcellular telephone.

A program storage device readable by a machine is provided, tangiblyembodying a program of instructions executable by the machine to performthe steps described herein. This could be, for example, a computerreadable medium.

The term memory block refers to any possible computer-related imagestorage structure known to those skilled in the art, including but notlimited to RAM, processor cache, hard drive, or combinations of those,including dynamic memory structures. Preferably, the methods andapplication program interface disclosed will be embodied in a program ofinstructions (not shown) either by coding in a high level language.

Any currently existing or future developed program storage devicesuitable for reading by a machine can be used to store the programsembodying the afore-described interface, methods and algorithms,including, but not limited to hard drives, floppy disks, digital tape,flash cards, compact discs, and DVD's. The program storage device cancomprise more than one device, such as two linked hard drives. Thisinvention is not limited to the particular hardware used herein, and anyhardware currently existing or developed in the future that permitsimage processing can be used.

As used herein, the terms tile histogram and regional histogram, tiletransfer function and regional histogram function, and interpolated tiletransfer function and image transfer function, are, respectively,synonyms.

Also, any element in a claim that does not explicitly state “means for”performing a specified function or “step for” performing a specifiedfunction, should not be interpreted as a “means” or “step” clause asspecified in 35 U.S.C. §112.

I claim:
 1. A method for filtering a digital image the methodcomprising: selecting a pixel from a first plurality of pixels, theselected pixel having a luminosity value and being within a neighborhoodcomprising a second plurality of pixels; and calculating a newluminosity value for the selected pixel as a function of the ratio of anumber of pixels in the neighborhood that fulfill a given condition to atotal number of pixels in the neighborhood, the condition comprising aluminance comparison against the selected pixel.
 2. The method of claim1, where the first plurality of pixels comprises the entire digitalimage.
 3. The method of claim 1, where an extent of the neighborhood isdefined by a distance function.
 4. The method of claim 3, where thedistance function comprises one or more of a spatial distance, a vectordifference, or a colormetric distance.
 5. The method of claim 3, wherethe number of pixels in the neighborhood that fulfill a given conditionin the ratio are weighted such that any pixels having a distance at orbelow a threshold distance to the selected pixel have a high weightadjustment value applied and any pixels having a distance above thethreshold distance to the selected pixel have a low weight adjustmentvalue applied.
 6. A nontransitory computer readable medium having storedthereon program instructions that, when executed by one or moreprocessors, cause the processors to perform operation including:selecting a pixel from a first plurality of pixels, the selected pixelhaving a luminosity value and being within a neighborhood comprising asecond plurality of pixels; and calculating a new luminosity value forthe selected pixel as a function of the ratio of a number of pixels inthe neighborhood that fulfill a given condition to a total number ofpixels in the neighborhood, the condition comprising a luminancecomparison against the selected pixel.
 7. The nontransitory computerreadable medium of claim 6, where the first plurality of pixelscomprises the entire digital image.
 8. The nontransitory computerreadable medium of claim 6, where an extent of the neighborhood isdefined by a distance function.
 9. The nontransitory computer readablemedium of claim 8, where the distance function comprises one or more ofa spatial distance, a vector difference, or a colormetric distance. 10.The nontransitory computer readable medium of claim 8, where the numberof pixels in the neighborhood that fulfill a given condition in theratio are weighted such that any pixels having a distance at or below athreshold distance to the selected pixel have a high weight adjustmentvalue applied and any pixels having a distance above the thresholddistance to the selected pixel have a low weight adjustment valueapplied.
 11. A system comprising: one or more processors; and anontransitory computer readable medium coupled to the one or moreprocessors and having stored thereon program instructions that, whenexecuted by the one or more processors, causes the one or moreprocessors to perform operations including: selecting a pixel from afirst plurality of pixels, the selected pixel having a luminosity valueand being within a neighborhood comprising a second plurality of pixels;and calculating a new luminosity value for the selected pixel as afunction of the ratio of a number of pixels in the neighborhood thatfulfill a given condition to a total number of pixels in theneighborhood, the condition comprising a luminance comparison againstthe selected pixel.
 12. The system of claim 11, where the firstplurality of pixels comprises the entire digital image.
 13. The systemof claim 11, where an extent of the neighborhood is defined by adistance function.
 14. The system of claim 13, where the distancefunction comprises one or more of a spatial distance, a vectordifference, or a colormetric distance.
 15. The system of claim 13, wherethe number of pixels in the neighborhood that fulfill a given conditionin the ratio are weighted such that any pixels having a distance at orbelow a threshold distance to the selected pixel have a high weightadjustment value applied and any pixels having a distance above thethreshold distance to the selected pixel have a low weight adjustmentvalue applied.